Material for plastic working, particularly that for forging, takes the form of a blank and is obtained by cutting a continuously cast rod or an extruded rod, or by blanking a rolled material.
Such material for forging is firstly made into a continuously cast rod of small diameter, which is obtained by continuous casting of molten metal, or into an ingot such as a billet for extrusion or a rolled slab.
The continuously cast rod of small diameter is subjected to homogenizing treatment, then to peeling for removing the cast surface and to cutting into blanks of predetermined thickness and predetermined length by use of a circular saw cutter to thereby produce a material for forging.
In the case of an extruded rod, the aforementioned billet is subjected to homogenizing treatment, then the treated material is extruded by an extruder, and drawn if necessary, and cut into blanks of predetermined thickness and predetermined length by use of a circular saw cutter to thereby produce a material for forging.
In the case of a rolled sheet, the aforementioned slab is subjected to homogenizing treatment, then the treated material is rolled with a hot rolling machine so as to yield a rolled sheet of desired thickness, followed by blanking to have a desired shape to thereby produce a material for forging.
The projection plane (cross-section) of any of these materials for forging is not necessarily round, but may be a centrally hollow circle, another shape resulting from profile extrusion, or any combination of these.
Material for plastic working has conventionally been produced through so-called direct chilling (casting employing forced cooling), in which cold water is applied directly to a cast ingot to thereby solidify the melt rapidly. This process attains a virtually uniform dispersion of solute elements.
However, when application of the material for plastic working to a functional member is desired, particularly in the case of application to a member required to have high strength and wear resistance, as typified by that made of high Si alloy, the material for plastic working must exhibit wear resistance in a specific portion at which the member being produced is brought into sliding contact with a counter member. Thus, wear resistance is not necessarily required for the entirety of the produced member.
For example, in the field of powder metallurgy, the relationship between the size of silicon grains and wear resistance is described in “Effect of Si grain size on wear resistance of Al—Si powder alloy” reported at the 73rd Fall Conference of the Japan Institute of Light Metals (November 1987).
Attempts have been made to obtain similar effects in the field of alloys produced from molten metal.
In relation to strength characteristics of alloys produced from molten metal, design of alloy components is directed to maintain the strength of a specific portion of a member to which a concentrated load is to be applied. However, in many cases, the strength tends to be imparted to the entirety of the member.
Thus, even when a required characteristic varies from portion to portion of a member, conventional material for plastic working has been used to give such a characteristic to portions other than the portion where the characteristic is required, because alloy components and metallographic alloy structure of the member are determined univocally so as to meet the requirement called for in relation to a specific portion.
Particularly in the case of an anti-wear alloy, added silicon (Si), which is added to serve as the secondary phase crystal alloy, crystallizes out in the course of solidification in the form of eutectic silicon or primary crystal silicon, and such crystallized silicon deteriorates workability of plastic deformation or machinability of the alloy.
The present invention has been made in order to overcome the above-described disadvantages, and its object is to provide a material for plastic working having an excellent function at a specific selected portion, by grading the inside compositional distribution and the metallographic morphology from the cooled face (bottom face) to the opposite face (top surface), as well as to provide a production method of the material.